Hydraulic fracturing is a process of pumping liquids down a well and into a subterranean formation at high rate and pressure, such that a fracture is formed in the rock around the well. After pumping a liquid volume sufficient to widen the fracture adequately, solid particles, called “proppant,” are added to the liquid. After pumping is completed, the well is opened for production of hydrocarbons. The production rate of fluid from the well is usually significantly increased after the fracturing treatment. Vast improvements in the hydraulic fracturing process have been developed since the process was originally patented in 1949 (U.S. Pat. Nos. 2,596,843 and 2,596,844).
The material first used for proppant in hydraulic fracturing of wells was silica sand. As wells became deeper, sand was found to have inadequate strength. In deep wells, stress of the earth causes the sand to crush and become much less effective in increasing the production rate of a well.
Synthetic proppant materials were developed to provide higher strength proppants. The original synthetic sintered proppant was sintered bauxite. In later years, a variety of ceramic raw materials have been used to make sintered ceramic proppants, including bauxite containing lesser amounts of alumina and clay minerals, such as kaolin. Generally, it has been found that the strength of ceramic particles increases with the amount of aluminum oxide (alumina) in the particle, all other factors remaining constant.
A general procedure for making synthetic proppant particles is to obtain the ceramic raw material, grind it to a fine powder, form it into pellets (called “green” pellets), and sinter the green pellets in a kiln. The final product is ceramic pellets in the size range suitable for proppants, from about 70 mesh to 12 mesh (0.008 inch to 0.067 inch in diameter). Different sizes of pellets are used depending on well conditions.
A variety of processes for forming the pellets of a proppant have been proposed. In early work, U.S. Pat. No. 4,427,068 describes a process for forming sintered ceramic pellets by adding dry powders of clay and alumina, bauxite, or mixtures to a high intensity mixer (hereinafter referred to as “dry mixing method”). Powdered fine grain ceramic starting ingredients (ceramic raw materials) are stirred to form a dry homogenous mixture. Then, sufficient water is added to cause agglomeration of the fine starting dust particles to form small composite spherical pellets from the powder. Continued mixing time is allowed in order to grow small pellets to the desired size. A broad range of sizes is produced during the pellet-forming stage. A preferred mixing device is obtained from Eirich Machines, Inc., and is known as the Eirich mixer. The resulting pellets are dried and sintered into the final proppant particles. Much of the ceramic proppant made in industry in past years has been made with this process of forming pellets.
U.S. Pat. No. 4,440,866 discloses an alternative process for producing pellets that are sintered to produce high strength pellets. A continuous spray/granulation of an aqueous aluminous ore suspension with binder is used to form granules that are subsequently sintered (hereinafter referred to as “spray fluidized bed method”). All steps of this process may be carried out in a continuous manner. An aqueous suspension containing the ceramic raw material is continuously atomized and fed into a layer of already partially dried small starting dust particles (often called seeds) that are fluidized in a stream of hot drying air. The aqueous ceramic raw material suspension is continuously sprayed and dried onto the seed particles until the desired finished green pellet diameter is achieved. Particles produced in this process have a size range that is less broad than those typically produced by the dry mixing method of U.S. Pat. No. 4,427,068 but are still of sufficient variation as to require further processing. Particles are continuously recovered from the fluidized layer and particles of the desired size are separated from oversized and undersized product fractions. Material is continuously recycled in the stream of drying air. This spray fluidized bed process has also been used to produce large amounts of ceramic proppants in industry.
The pellet-forming methods described above have intrinsic limitations. The dry mixing process produces an extremely wide range of green pellet sizes due to the random nature of the agitation of the rotor and pan. The spray fluidized bed process produces a somewhat tighter green pellet size distribution but still a much wider distribution than desired. These processes require extensive screening and recycling during the manufacturing process. Under the best manufacturing conditions about 30% of green pellets must be recycled through the pellet-forming process. Both the dry mixing and spray fluidized bed processes also produce a random distribution of pore sizes in pellets, including a small percentage of very large pores that significantly degrade pellet strength. Strength of the sintered pellets is a primary consideration, because if the pellets break under high stress in a fracture, the flow capacity of the fracture is decreased and the hydraulic fracturing treatment is less effective. The sphericity and surface smoothness of particles produced by these processes are also important, with high sphericity and a very smooth surface traditionally being most desirable. All of these characteristics are strongly affected by the pellet-forming method.
U.S. Pub. No. 2006/0016598 discloses a list of pellet-forming techniques that may be used for ceramic proppant formation, including agglomeration, spray granulation, wet granulation, extruding and pelletizing, vibration induced dripping according to U.S. Pat. No. 5,500,162, spray nozzle-formed droplets and selective agglomeration. U.S. Pat. No. 5,500,162 discloses producing microspheres by vibration-provoked dripping of a chemical solution through a nozzle plate, wherein the falling drops form an envelope surrounded from all sides by flowing reaction gas. The liquid chemical solution has no or low (i.e., 20% or less) solid particles at the time it enters the nozzle plate, exits the nozzle plate, and passes through the first free fall section. The reaction gas is required to cause the precipitation (gelling) of small solid particles (typically sub-micron) in the liquid drops as they fall through the second free fall zone, and thereafter fall into a reaction liquid to further gel. The reaction gas is necessary to cause the liquid to partially gel prior to entering the reaction liquid, and the droplets are decelerated into the liquid through a foam or the reaction liquid is directed onto the falling drops tangentially in the same direction in which the droplets are falling. These two features of falling through reaction gas and decelerating the droplets into foam are required to insure the droplets are partially gelled during a sol-gel reaction and therefore not deformed, for example flattened, when they strike the reaction liquid. The reaction gas is sucked away inside or outside the envelope. The method according to the invention can be used to produce, for example, aluminum oxide spheres up to the diameter of 5 mm.
Vibration-induced dripping, herein called “drip casting,” was originally developed to produce nuclear fuel pellets. Since then it has been adapted to produce a very wide variety of metal and ceramic “microspheres,” such as grinding media and catalyst supports. Primarily, it has been used in the food and pharmaceuticals industries. The drip casting process is described on the website and in sales literature of Brace GmbH. Examples of microspheres formed by drip casting of different materials are also provided. U.S. Pat. No. 6,197,073 discloses a process for producing aluminum oxide beads from an acid aluminum oxide sol or acid aluminum oxide suspension by flowing the suspension through a vibrating nozzle plate to form droplets and pre-solidifying the droplets with gaseous ammonia and then coagulating the droplets in an ammonia solution. The mechanical strength of ceramic particles formed by sintering the drip cast particles was not a factor in any of the materials used in these references.
It is known that to produce ceramic proppant particles having maximum strength for a given ceramic material, the particles must contain minimum porosity, and the pores present must be kept as small as possible, since the strength of a given proppant particle is limited by its largest pore. What is needed is a method of forming green ceramic particles that can be fired to have reduced pore size and therefore maximum strength for use as a proppant. Preferably, the particles should be spherical, have a smooth surface and have uniform size. A method for forming the green pellets without recycling of the undesired size fraction of green ceramic pellets is also needed.